1. Field of the Invention
The invention relates to devices and methods for film archival based upon digitization of dye densities of the film. In particular, the devices and methods of the invention use the analytical densities of the dyes and the dye concentrations at each pixel to determine the true color of a particular pixel where such color value is not necessarily influenced by aging of the film substrate or chemical degradation of the color dyes used.
2. Background Art
In prior art systems, the color of a transmissive sample of film or other material is generally expressed as the sum of projection onto each of three primary colors, such that: EQU Arbitrary Color=R.multidot.(Primary 1)+G.multidot.(Primary 2)+B.multidot.(Primary 3) Equation (1)
where R, G, and B are the projections of the sample's color on the primaries. If the sample is illuminated by a light source with a power spectrum s(.lambda.) and has a spectral transmission of .tau.(.lambda.) where .lambda. represents the wavelength of light, then the projection onto each primary is given by: ##EQU1## where r(.lambda.), g(.lambda.), and b(.lambda.) are the projections of the color of a pure spectral line at wavelength .lambda. onto the three chosen primaries, and the integral is over all visible wavelengths.
To measure color, it is necessary to construct filters having spectral transmission curves proportional to the functions r, g, and b, so that the RGB values can be deduced by simply measuring the power incident on an optical detector from the source s(.lambda.) after passing through the sample and the appropriate filter. If the detectors used do not have perfectly flat spectral response, then they must be divided out of r(.lambda.), g(.lambda.), and b(.lambda.) to achieve the appropriate filter transmissions.
Furthermore, the projections on each primary must necessarily be non-negative when using this type of three-filter color measurement method. Moreover, negative projections can be achieved only by using a fourth filter to supply a subtractive constant. Nonetheless, in prior art systems, measuring all visible colors with three filters requires that the primaries be chosen so that the projections of all visible colors are non-negative. For instance, the CIE standard primaries X, Y, and Z represent such a set and their spectral functions are shown in FIG. 1.
Constructing a set of filters which accurately match these tristimulus functions is expensive and represents a significant shortcoming of heretofore known systems. These typical color measurement systems use three filters which only pass radiation in the wavelength bands ("wavebands") which correspond generally to the humps of the tristimulus curves. Therefore, the transmission curves do not correspond to spectral projection onto actual primaries, but only approximate them.
For these reasons, typical color measurement devices (densitometers and scanners) have inherent errors in their color measurements. These inherent errors lead to at least three major disadvantages of prior systems. First, high accuracy requires that the transmission of the filters accurately correspond to the tristimulus curves (or the projection of spectral colors on some other set of primaries) divided by the responsivity of the detectors. Second, the measurement of color depends upon the light source, s(.lambda.). Finally, if restoration of faded colors is to be accomplished, no global transformation to calculate the pre-fading primary projection values from the faded primary projection values exists.
As noted above, the first disadvantage of prior systems is the requirement for high accuracy of accurate filter correspondence to the tristimulus curves divided by detector responsivity. While this disadvantage can be overcome by careful construction of the filters, accurate broadband filters are difficult to manufacture. Furthermore, any variation in responsivity from detector to detector must be accounted for by making customized filters for each detector. Therefore, although in principle this disadvantage can be overcome, in practice the limitation reduces the achievable accuracy for a system where a reasonable cost is necessary.
The second disadvantage, the dependence of the measured color on the light source, is important for reasons of both aesthetics and accuracy. Although most films are manufactured for viewing under a specified light source, the appropriate source to be used for any film is subjective, and source preferences may change with the development of new sources. Unfortunately, in conventional methods, once a light source is chosen for the digitization process, there is no simple, global transformation of the color measured using one light source which corresponds to that measured using another source. Moreover, any measurement process which depends on a light source is limited in accuracy by intensity drifts in the light source.
Finally, in conventional systems, the dependence on the light source can be eliminated by measuring and recording the actual spectral transmission of the film .tau.(.lambda.), rather than the color. However, the elimination of the source dependence is inefficient for several reasons. Elimination of light source dependence is normally accomplished by measuring the light intensity passed by the film in each spectral band and then dividing out the intensity of the light source in that band. If the source radiation is measured simultaneously (or at least nearly simultaneously) with the radiation passed by the film, this method is highly accurate. The problem with measuring and recording the transmission in this way is that it requires two detections for each spectral band (and thus many detectors or long scan times). Also, recording the transmission requires more information storage than is required for recording color. However, once the transmission is measured and stored, the color of the film can be calculated from equation 2, below, for any assumed light source.
Therefore, it is highly desirable to provide a system of color measurement which accurately models the tristimulus curves. Furthermore, it is highly desirable to provide a system of color measurement which is independent of the light source and which requires little information storage overhead. Finally, it is highly desirable to provide a method of restoring faded colors where a single, global transformation can accomplish the restoration in a very efficient manner.